IL-6 is an inflammatory cytokine that is elevated in several autoimmune and inflammatory disorders, but also in a number of cancers. Inhibition of IL-6 by an anti-IL-6R antibody, tocilizumab, is tremendously therapeutic in this rheumatoid arthritis. This highlights the pathogenic role of IL-6 in this disease, and likely several other inflammatory disorders, and hence underscores the need to fully understand IL-6 function. IL-6 plays a critical role in determining the type of cytokines produced by effector CD4 T cells. Most of these functions of IL-6 have been assigned to a regulatory role on gene expression through the Stat3 transcription factor. However, recent studies have revealed that in addition to being localized in the cytosol and nucleus, Stat3 is also present in the mitochondria where it regulates the mitochondrial respiratory chain through association with Complex I. leaving a new area of research open to understand the mechanism, impact and development of novel therapies. We have recently identified a novel mechanism by which IL-6 regulates CD4 cell function using this alternative pathway. Using mouse models, we have shown that IL-6 sustains high mitochondrial membrane potential (MMP) late during in vitro activation of CD4 cells through Stat3, independently of its transcription activity. Mitochondrial hyperpolarization caused by IL-6 however is uncoupled from the production of ATP by oxidative phosphorylation. Instead, IL-6 uses the elevated MMP to promote mitochondrial Ca2+ accumulation in CD4 cells. Mitochondria are emerging as the primary subcellular Ca2+ stores that regulate cytosolic Ca2+ homeostasis and mediate the delivery of extracellular Ca2+ and Ca2+ form endoplasmic reticulum. We have shown that IL-6 uses mitochondrial Ca2+ to sustain elevated cytosolic Ca2+ levels in effector CD4 cells, and this contributes to a prolonged production of cytokines (IL-21 and IL-4) by these cells in vitro. To demonstrate the relevance of these novel findings, we now propose in this application to investigate the presence of this IL-6-mediated pathway in CD4 cells in vivo using mouse models, in human CD4 cells in vitro and in vivo, and its impact on in vivo cytokine production as well as in migration of CD4 cells since Ca2+ is an essential mediator for cell motility. Specifically we propose to investigate: 1) the regulation of MMP and mitochondrial Ca2+ through Stat3 by IL-6 in CD4 cells in vivo, and its impact on cytokine production; 2) whether IL-6 regulates migration of CD4 cells by enhancing mitochondrial Ca2+ homeostasis. 3) the effect of IL-6 on MMP and mitochondrial Ca2+ in human CD4 cells in vitro and in vivo, and the contribution of this pathway to cytokine production and/or migration. The studies can reveal a new mechanism for IL-6 in chronic diseases where CD4 cells play an important role, and they can lead to the development of new inhibitors targeting the mitochondrial Ca2+ pathway, as well as a redesign of inhibitors for Stat3 that also target mitochondrial Stat3.